The Canvas inbox feature is great for reaching many students at once unless you have an account-level Canvas group that combines students from multiple sections of the same course. The inbox feature allows you to easily find and message individual members of the group. As a Group Leader, there is no "message all members of the group" option when composing a new message.
It would be great to have the ability (at least as a Group Leader) to message all members of the group. Currently the Group Leader only has the power to change the name of the group.